第三讲宇宙中的暗物质和暗能量

文章对暗物质粒子的候选者和宇宙中暗能量的研究现状作一简单介绍．     

Dark matter and dark energy of the universe

A possibility of existing spheres filled with a uniform constant scalar field in the Universe is shown. These spheres can act as “dark matter” and can be responsible for a decreasing behavior of the “ rotational” curved galaxies observed. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 9–19, April, 2006.     

Quantum graviton creation in a model universe

Quantization in the spatially inhomogeneous, anisotropic, Gowdy three-torus solution of Einstein's equations leads to the production of gravitons from empty space. The creation of pairs of gravitons occurs only in wave modes with wavelength exceeding the horizon size at an initial time. The final number of created gravitons in any mode is proportional to the number of causally unconnected regions at the initial time over the wavelength of that mode. At large times, graviton number is well defined since the solution is in WKB form. The creation process produces the anisotropic collisionless radiation identical to that discussed by Doroshkevich, Zel'dovich, and Novikov which characterizes the large time classical solution. Near the singularity, the model behaves like an empty Bianchi Type I universe at each point in space (local Kasner). The canonical methods of Arnowitt, Deser, and Misner yield a reduced Hamiltonian from which the classical equations of motion are obtained. The quantization of the rapidly varying gravitational field component resembles the procedures used by Parker or Zel'dovich et al. to study particle creation in curved spacetime.     

Dark energy, cosmic coincidence and future singularity of the universe

Dark energy model with the equation of state $p_{DE} =-\rho _{DE} -A\rho _{DE}^\alpha $ , is characterised by four finite life time future singularity of the universe for different values of the parameter $A$ and $\alpha $ [Nojiri et al. in Phys Rev D 71:063004, 2005]. Since from the matter dominated era to the dark energy dominated era the ratio of the dark energy density to the matter energy density increases as the universe expand for these future singularities, the universe passes through a significant time when the dark energy density and the matter energy density are nearly comparable. Considering $\frac{1}{r_0 }<r=\frac{\rho _{DE} }{\rho _M }<r_0 $ , where $r_0$ is any fixed ratio, we calculate the fraction of total life time of the universe when the universe passes through the coincidental stage for these singularities. It has been found that the fractional time varies as $\alpha $ varies within the range for which these finite life time future singularities occur and the fraction is smaller for smaller values of $r_0 $ . Importance of the fractional time and observational limits onto the values of the parameter $A$ and $\alpha $ has also been discussed.     

Cosmological constraints on the graviton mass in RTG

The Friedmann cosmological scenario in RTG (without inflation) is considered. The joint maximum-likelihood analysis of data on type Ia supernovae, the shift parameter of microwave radiation, and baryon acoustic oscillations from the Sloan catalogue of red galaxies provided tight fit constraints on the graviton mass and the space curvature in GR. It is demonstrated that the confidence interval for the graviton mass extends indefinitely if the quintessence parameter tends to zero. These conclusions are valid if the present scale factor a ₀ >(2)^?1/6= 0.89. At a ₀ <(2)^?1/6, a tight constraint on the graviton mass was derived from these observational data: m < 10^–83 g. This implies that terms with the graviton mass may be neglected (with the exception of solutions of the black-hole type) in the gravitational field equations in a broad range of redshifts (0 < z < 10¹⁵).     

Dark matter and the baryon asymmetry of the universe

We present a mechanism to generate the baryon asymmetry of the Universe which preserves the net baryon number created in the big bang. If dark matter particles carry baryon number Bx, and sigmaxannih相似文献   

Gravitational energy in the expanding universe

The role of gravitational energy in the evolution of the universe is examined. In co-moving coordinates, calculation of the Landau-Lifshitz pseudotensor for FRW models reveals that: (i) the total energy of a spatially closed universe irrespective of the equation of state of the cosmic fluid is zero at all times, (ii) the total energy enclosed within any finite volume of the spatially flat universe is zero at all times, (iii) during inflation the vacuum energy driving the accelerated expansion and ultimately responsible for the creation of matter (radiation) in the universe, is drawn from the energy of the gravitational field. In a similar fashion, certain cosmological models which abandon adiabaticity by allowing for particle creation, use the gravitational energy directly as an energy source.     

The energy of the universe

The energy of the universe, including the energy of the matter and that of the gravitational field, is investigated with the help of the Einstein gravitational pseudo-tensor. It is found that the total energy vanishes.     

Vacuum energy and the universe in special relativity

The problem of the cosmological constant and vacuum energy is usually thought of as the subject of general relativity. However, vacuum energy is important for the Universe even in the absence of gravity, i.e., in the case when Newton’s constant G is exactly zero, G=0. We discuss the response of the vacuum energy to the perturbations of the quantum vacuum in special relativity and find that, as in general relativity, the vacuum energy density is on the order of the energy density of matter. In general relativity, the dependence of the vacuum energy on the equation of state of matter does not contain G and thus is valid in the limit G→0. However, the result obtained for the vacuum energy in a world without gravity, i.e., when G=0 exactly, is different.     

Dark energy and gravity

I review the problem of dark energy focussing on cosmological constant as the candidate and discuss what it tells us regarding the nature of gravity. Section 1 briefly overviews the currently popular “concordance cosmology” and summarizes the evidence for dark energy. It also provides the observational and theoretical arguments in favour of the cosmological constant as a candidate and emphasizes why no other approach really solves the conceptual problems usually attributed to cosmological constant. Section 2 describes some of the approaches to understand the nature of the cosmological constant and attempts to extract certain key ingredients which must be present in any viable solution. In the conventional approach, the equations of motion for matter fields are invariant under the shift of the matter Lagrangian by a constant while gravity breaks this symmetry. I argue that until the gravity is made to respect this symmetry, one cannot obtain a satisfactory solution to the cosmological constant problem. Hence cosmological constant problem essentially has to do with our understanding of the nature of gravity. Section 3 discusses such an alternative perspective on gravity in which the gravitational interaction—described in terms of a metric on a smooth spacetime—is an emergent, long wavelength phenomenon, and can be described in terms of an effective theory using an action associated with normalized vectors in the spacetime. This action is explicitly invariant under the shift of the matter energy momentum tensor T _ab → T _ab + Λ _gab and any bulk cosmological constant can be gauged away. Extremizing this action leads to an equation determining the background geometry which gives Einstein’s theory at the lowest order with Lanczos–Lovelock type corrections. In this approach, the observed value of the cosmological constant has to arise from the energy fluctuations of degrees of freedom located in the boundary of a spacetime region.     

Dark energy and the BOOMERANG data

The recent high-quality BOOMERANG data allow the testing of many competing cosmological models. Here I present a seven-parameter likelihood analysis of dark energy models with exponential potential and explicit coupling to dark matter. The BOOMERANG data constrain the dimensionless coupling beta to be smaller than 0.1, an order of magnitude better than previous limits. In terms of the constant xi of nonminimally coupled theories, this amounts to xi<0.01. On the other hand, BOOMERANG does not have enough sensitivity to put constraints on the potential slope.     

Dark energy versus modified gravity: Impacts on measuring neutrino mass

In this paper,we make a comparison for the impacts of smooth dynamical dark energy,modified gravity,and interacting dark energy on the cosmological constraints on the total mass of active neutrinos.For definiteness,we consider theΛCDM model,the w CDM model,the f(R)model,and two typical interacting vacuum energy models,i.e.,the IΛCDM1 model with Q=βHρc and the IΛCDM2 model with Q=βHρΛ.In the cosmological fits,we use the Planck 2015 temperature and polarization data,in combination with other low-redshift observations including the baryon acoustic oscillations,the type Ia supernovae,the Hubble constant measurement,and the large-scale structure observations,such as the weak lensing as well as the redshift-space distortions.Besides,the Planck lensing measurement is also employed in this work.We find that,the w CDM model favors a higher upper limit on the neutrino mass compared to theΛCDM model,while the upper limit in the f(R)model is similar with that in theΛCDM model.For the interacting vacuum energy models,the IΛCDM1 model favors a higher upper limit on neutrino mass,while the IΛCDM2 model favors an identical neutrino mass with the case ofΛCDM.     

The hidden mass of the universe

It is generally accepted that the hidden mass of the Universe consists of massive neutrinos or other hypothetical particles (axions, photinos, etc. We assert that there is no basis for such hypotheses. Even if the neutrino possesses a mass, it would be too small, and despite the great efforts to observe the other particles, the results have been negative. If the mass distribution law f(M) M^–2 established for meteors meteorites and asteroids in the range between 10^–12 and 10²⁰ g is extended to the Universe as a whole, one obtains values for the density of the luminous matter, transparency of the galaxies and of the Universe which agree with those observed. It is assumed that the primordial deuterium was burnt up during continuous star formation, and the deuterium observed at present is of a secondary origin. It is shown that very probably the metallicity of stars of the solar type may in reality be tens of times greater than that observed in the photosphere which reflects only the metallicity of a convection layer with a thickness of less than 0.2 of the radius. The difficulties that arise if it is assumed that the dark matter consists of hypothetical noninteracting particles are mentioned: at t 10¹³ sec there cannot be any perturbations of the density of particles with mc²<20 eV at a level of 10^–4 (absence of fluctuations of the microwave background radiation); particles with mc²>10³ eV should decay during a period of 10⁸–10⁹ years and thus distort significantly the t(T) dependence; particles with mc²>10⁵ eV strongly reduce the thermonuclear synthesis time and consequently (D/H)>10^–3 and (⁴HeH)<0.2.I. V. Kurchatov Institute of Atomic Energy, Moscow. Translated from Izvestiya Vysshikh Uchebnikh Zavedenii, Fizika, No. 1, pp. 13–22, January, 1993.     

Original neutrino fluxes and hidden mass in the universe

The status of neutrino experiments in connection with the fundamental problem of searches for a signal from dark matter is discussed. Limits on the magnitude of the effect of dark-matter-particle annihilation in the Sun that were obtained with neutrino telescopes are presented. In particular, the first results from the NT-200 Baikal Deep Underwater Neutrino Telescope are described.     

Hubble induced mass in radiation-dominated universe

We reconsider the effective mass of a scalar field which interact with visible sector via Planck-suppressed coupling in supergravity framework. We focus on the radiation-dominated (RD) era after inflation. In this era, the effective mass is given by thermal average of interaction terms. To make our analysis clear, we rely on Kadanoff–Baym equations to evaluate the thermal average. We find that, in RD era, a scalar field acquires the effective mass of the order of H.     

暗物质与暗能量

美国《science》期刊所评2003年十大科学成就中将“证实了宇宙主要由神秘的暗物质和暗能量组成”列为首项：“宇宙是平直的；宇宙中存在4．4％重子物质，22．6％暗物质和73％暗能量．”这是科学家们分析了威尔金森微波各向异性探测器(Wilkinson Mi-     

Perspectives on the energy of the universe

Recent progress in computing the energy of the universe including the gravitational contribution is discussed. Various issues are raised including symmetries, energy localization and observational verification.